Field of the Invention
The present invention relates to improvements in processes and apparatus for producing synthesis gas, or syngas, from light hydrocarbon gas such as methane or natural gas by the partial oxidation and autothermal steam reforming thereof. Such syngas, comprising a mixture of carbon monoxide and hydrogen, is useful for the preparation of a variety of other valuable chemical compounds, such as by application of the Fischer-Tropsch process.
The combustion stoichiometry of methane gas at 1000.degree. F. is highly exothermic and produces CO.sub.2 and H.sub.2 O according to the following reaction: EQU CH.sub.4 +2O.sub.2 .fwdarw.CO.sub.2 +2H.sub.2 O
(-190.3kcal/g mol CH.sub.4).
The formed gases are not useful for the production of valuable chemical compounds, and the high temperatures generated present problems with respect to reactors and catalysts which would be required to produce valuable products from the formed gases.
It is known to produce useful gases, known as synthesis gases or syngases, by partial oxidation of methane and other light hydrocarbon gases, by steam or CO2 reforming of methane and other light hydrocarbon gases, or by some combination of these two chemistries. The partial oxidation reaction of methane is a less highly exothermic reaction which, depending upon the relative proportions of the methane and oxygen and the reaction conditions, can proceed according to the following stoichiometry: EQU 2CH.sub.4 +2O.sub.2 =2CO+2H.sub.2 +2H.sub.2 O
(-64kcal/g mol CH.sub.4.) EQU 2CH.sub.4 +1.50.sub.2 =2CO+3H.sub.2 +1H.sub.2 O
(-34.9 kcal/g mol CH.sub.4.)
or EQU 2CH.sub.4 +O.sub.2 =2CO+4H.sub.2 +OH.sub.2 O
(-5.7kcal/g mol CH.sub.4.)
It is most desirable to enable the partial oxidation reaction to proceed according to the latter reaction in order to produce the most valuable syngas and minimize the amount of heat produced, thereby protecting the apparatus and the catalyst bed, and to reduce the formation of steam, thereby increasing the yield of hydrogen and carbon monoxide, and enabling the steam-reforming reaction to convert any steam and hydrogen into useful syngas components.
Conventional syngas-generating processes include the gas phase partial oxidation process (GPOX), the autothermal reforming process (ATR), the fluid bed syngas generation process (FBSG), the catalytic partial oxidation process (CPO) and various processes for steam reforming. Each of these processes has advantages and disadvantages when compared to each other.
The GPOX process, illustrated for example by U.S. Pat. No. 5,292,246; UK Application GB 2,202,321A and EPO Application 0 312 133, involves the oxidation of the feed hydrocarbon gaseous, liquid or solid form, in the gas phase rather than on a catalyst surface. The individual components are introduced at a burner where they meet in a diffusion flame, which produces over-oxidation and excessive heat generation. The gas may be preheated and pressurized, to reduce the reaction time.
The ATR process and the FBSG process involve a combination of gas phase partial oxidation and steam reforming chemistry.
In the conventional ATR process, illustrated for example by U.S. Pat. No. 5,492,649 and Canadian Application 2,153,304, the hydrocarbon feed and the oxygen feed, and optionally steam, are heated, and mixed in a diffusion flame at the outlet of a single large coaxial burner or injector which discharges into a gas phase oxidation zone. The gases are reacted in the gas phase in the partial oxidation combustion zone, and then flow into a large bed of steam reforming catalyst, such as large catalyst pellets, or a monolithic body or ceramic foam, to catalyze the steam reforming reaction. The entire hydrocarbon conversion is completed by a single reactor aided by internal combustion. The burner is the key element because it mixes the feedstreams in a turbulent diffusion flame. The reaction products are introduced to the fixed bed catalyst zone, preferably of large catalyst pellets, at high temperatures from the combustion zone, due to the over-oxidation which occurs in the diffusion flame of the burner, where the oxygen and hydrocarbon gas meet. The diffusion flame includes oxygen-rich and hydrocarbon-rich zones. These result in both complete combustion and substantially higher temperatures, in the oxygen-rich zones, and hydrocarbon cracking and soot-formation, in the hydrocarbon-rich zones.
In the ATR process, the gases are intended to react before they reach the catalyst, i.e., the oxidation chemistry occurs in the gas phase, and only the steam reforming chemistry occurs in the catalytic bed. In fact, long residence times are required because diffusion flames are initiated with a large amount of over-oxidation, accompanied by a large amount of heat. Thus, time is required for the relatively slow, endothermic gas phase steam reforming reactions to cool the gas enough to prevent thermal damage to the catalyst.
The gas phase partial oxidation and steam reforming chemistry employed in the FBSG and the Autothermal Reforming (ATR) process have very similar material balance when using similar feed. However, ATR is limited in size by the scaleability of its injector design, and the more-scalable FBSG is economically debited by the cost of fluid solids and dust cleanup and by the expense of replacing agglomerated and/or eroded catalyst. The dust comprises catalyst fines due to catalyst attrition in the bed, and these fines are expensive to clean out of the syngas. While the chemistry is correct, these two processes have significant drawbacks. Both require very large reactors. For FBSG there is a significant expense in fluid solids management. For Autothermal Reforming there is a large and problematic methane/oxygen feed nozzle.
In the autothermal reforming process, the methane and oxygen-containing gases are mixed and reacted in a diffusion flame, and the oxidized effluent is passed into a steam reforming zone for steam reforming of the effluent in the presence of a fixed arrangement of a conventional steam reforming catalyst, such as a fixed catalyst bed or a ceramic foam or monolith carrier impregnated with a steam reforming catalyst.
The high temperature in the catalytic reforming zone places great demands on the reforming catalyst, which must withstand these conditions and be capable of substantially retaining its catalytic activity and stability over many years of use.
Conventional steam-reforming catalysts, or autothermal or combined reforming catalyst, can be described as being selected from the group consisting of uranium, Group VII metals, and Group VIII noble and non-noble metals. Metals may be used in combination with each other and with other metals such as lanthanum and cerium. The metals are generally supported on thermally-stable inorganic refractory oxides. Preferred catalyst metals are the Group VIII metals, particularly nickel. In the case of nickel, any nickel-containing material is useful, e.g. nickel supported on alpha alumina, nickel aluminate materials, nickel oxide, and preferably a supported nickel containing material.
Support materials include .alpha.-alumina, aluminosilicates, cement, and magnesia. Alumina materials, particularly fused tabular alumina are particularly useful as catalyst support. Preferred catalyst supports may be Group II metal oxides, rare earth oxides, .alpha.-alumina, modified .alpha.-aluminas, .alpha.-alumina-containingoxides, hexa-aluminates, Ca-aluminate, or magnesium-alumina spinel. In some cases, catalysts are stabilized by addition of a binder, for example calcium aluminum oxide. Silicon Dioxide level in the catalyst is preferred to be maintained at a very low level, e.g. less than 0.3 wt % to avoid volatilization and fouling of downstream equipment.
The shape of the catalyst carrier particles may vary considerably. Raschig rings 16 mm in diameter and height having a single 6-8 mm hole in the middle are well known in the art. Other forms, such as saddles, stars, and spoked wheels are commercially available.
According to the autothermal steam reforming process of U.S. Pat. No. 5,492,649 the production of high amounts of carbon or soot in the diffusion flame oxidation step is avoided by mixing the methane gas with the oxidizer gas while swirling the latter at the injection nozzle to provide a large number of mixing points in the diffusion flame. However, such process still produces the partial oxidation reaction in a diffusion flame, which results in overoxidation and an excessively high temperature effluent which can damage the steam reforming catalyst and the face of the injector.
According to Canadian Application 2,153,304, the formation of soot is avoided or reduced by reducing the molar feed ratio of steam to carbon to increase the steam reforming temperature between about 1100.degree. to 1300.degree. C., and/or by introducing the gaseous hydrocarbon feed in increments.